In the commercial production of phosphoric acid, ground phosphate rock is metered to a digestion circuit along with weak (about 18-20% P205) phosphoric acid recycled from the filter plus recycled slurry and fresh sulphuric acid. This mixture is stirred and recycled until substantially all the rock has been digested.
Excess slurry is overflowed to a filter feed tank, thence to a filter where gypsum is separated from the strong (about 28-30% P205) phosphoric acid. Wash water, in countercurrent washing steps, displaces the residual acid from the cake which then becomes the source of the weak recycle phosphoric acid. Reference is made to Chapter 16 of Volume II, "Phosphorus And Its Compounds", edited by John R. Wazer, published by Interscience Publishers, 1961.
Phosphate rock usually contains about 1 to 10% CO2, generally 4 to 6% plus 0.1 to 4.0% organic material, generally 0.2 to 0.8%. As the rock is attacked by the phosphoric acid-sulphuric acid mixture, considerable foam is generated and, if allowed to remain, becomes stable and will overflow the digestion tanks. To control this condition several techniques have been applied such as mechanical foam breakers, steam jets, water jets, and air jets played on the surface of the foam. It is said that this causes the bubbles to break and the liquified foam then is returned into the body of the slurry. The most accepted method, however, has been to add surfactants or commercial defoamers which cause the bubbles to collapse and the liquified foam is returned into the body of the slurry.